Instruments for recording electric power parameters are well known. These instruments typically record power parameters such as volts, amps, or watts, as a function of time. Traditionally, the recording medium was a strip of paper that advanced steadily as the instrument drew a line representative of the parameter's value. In more modern instruments, the measured parameters are converted to digital values which are subsequently stored in digital memories for later extraction, analysis, communication, or presentation.
There are two types of digital memory commonly used for this purpose: semiconductor RAM (random access memory) and disk drives.
Semiconductor RAM has the advantage of operating over a wide temperature range. For example, the TMS4C1024 RAM, available from Texas Instruments of Houston, Tex. is specified for operating between 0 degrees centigrade and 70 degrees centigrade, and for storage between +65 degrees centigrade and +150 degrees centigrade.
Disk drives operate over a much narrower temperature range. For example, the Model GO12A 120 Megabyte Hard Disk Drive available from Quantum Corporation in Milpitas, Calif. is specified to operate only from 5 degrees centigrade to 55 degrees centigrade, although it may be stored at temperatures from -40 degrees centigrade to +70 degrees centigrade. Most electronic devices operate over a wider temperature range than disk drives; consequently, if an electronic instrument incorporates a disk drive, it is common for the disk drive operating temperature specifications to determine the operating temperature specifications of the instrument as a whole.
Disk drives have the advantage of being more than an order of magnitude less expensive than semiconductor RAM per kilobyte of storage.
Instruments that record power parameters often consume sufficient electric power to make operating at low outside temperatures fairly easy; the power consumed by the instrument raises its temperature, and some instruments even incorporate a small heater. For this reason, operation at low outside temperatures is not normally a problem for electric power recording instruments.
However, at high outside temperatures it is difficult to inexpensively and reliably cool the instrument. For example, for power-pole mounted instruments, ambient outside temperatures of 40 degrees centigrade are common. Heat rise from direct solar incidence on the instrument enclosure commonly contributes an additional 10 degrees centigrade or more, and internal power dissipation in the instrument commonly contributes another additional 15 degrees centigrade or more. Internal operating temperature therefore commonly reaches 65 degrees centigrade. This temperature is higher than temperatures at which disk drives can commonly operate. Consequently, in prior art in power recording instruments designed to reliably operate outdoors, expensive semiconductor RAM memories must be used instead of disk drives.